Very commonly samples are taken of a patient's blood for laboratory tests. This blood is obtained from the patient's veins by means of a hypodermic needle inserted into a suitable superficial vein which has been previously distended by means of a tourniquet. The required quantity of blood is drawn into the syringe by pulling on the barrel of the instrument.
More recently the conventional syringe has been partially replaced by a technique employing a series of preevacuated containers. When such a system is used, a single needle penetrates the vein and a series of evacuated containers are sequentially connected thereto for drawing a number of blood samples for different analyses. The individual containers are advantageous since they are disposable and presterilized and can secure consistent blood volume.
Such an arrangement can have problems, however, since the angle of vein penetration is sometimes awkward when the container is large. In some instances the puncture needle has been placed eccentrically on the container to minimize this problem. When the individual preevacuated containers are changed there is a significant chance that the puncture needle will be displaced or will penetrate the opposite wall of the vein. In either case this necessitates an additional puncture and often results in hematoma formation. The condition of the patient's vein and the skill of the operator are important factors in determining whether this may occur.
In addition, the preevacuated collection tubes often contain a substance such as an anticoagulant or blood preservative which can have significant adverse effects on the patient if it is aspirated into the patient's bloodstream. Such could happen, for example, if the container has developed a leak and the vacuum has been lost. This condition cannot be ascertained by simple inspection and careless work on the part of the person taking the sample could lead to undesirable or toxic substances entering the patient's bloodstream.
Blood is composed of a solid component, namely the red blood cells, platelets, and white cells and a fluid component or plasma. These components can be separated and depending on the tests being carried out it is desirable to select any of three or four fractions of the blood sample. Whole blood is desirable for red blood cell counts, white blood cell counts, platelet counts and erythrocyte sedimentation rates. When whole blood is desired the blood is collected into an anticoagulant substance such as calcium oxalate or heparin. Plasma is sampled for measurement of blood albumin, globulin, fibrinogen, and protein electrophoresis, and is employed in cross matching of blood. Plasma is the liquid portion of the blood obtained after centrifuging anticoagulated blood. Serum is the fluid left after blood has coagulated. It is essentially plasma without its proteins which remain in the coagulated portion. In addition, it may be desirable for some tests to obtain a sample rich in the so-called buffy layer which remains after centrifuging as a thin layer rich in white blood cells atop the heavier red blood cells.
Due to different concentrations of body chemicals inside the blood cells and outside in the plasma or serum it is essential for most tests to separate the cells from the plasma rather quickly after blood has been withdrawn from veins and before the contents of the cells leak out into the plasma.
Ordinarily when blood samples are taken, analysis is made on whole blood or on the serum remaining as a supernatant liquid following blood coagulation, or alternatively on the liquid plasma which overlies precipitated solid elements of the blood. It is desirable to provide a technique for safely and quickly collecting blood from the patient, and separating blood into the desired fractions. Various tests require samples of predetermined volume and it is helpful to the laboratory technician to have simple means for rapidly and reliably taking samples of measured volume.